ML22033A006
| ML22033A006 | |
| Person / Time | |
|---|---|
| Issue date: | 03/10/2022 |
| From: | NRC/NRR/DNRL |
| To: | |
| References | |
| Download: ML22033A006 (8) | |
Text
Reactor Pressure Vessel Embrittlement Monitoring and Prediction in Long-Term Operation Office of Nuclear Reactor Regulation The embrittlement trend curve (ETC) provides estimates of change in fracture toughness (T,RTNDT) as a function of fluence.
Surveillance capsule testing generates monitoring data to ensure the ETC predicts plant-specific behavior properly.
Together, they are used to determine pressure-temperature (PT) limits for normal operation and to demonstrate sufficient toughness for protection from pressurized thermal shock events.
ART = Adjusted Reference Temperature
Embrittlement Uncertainty Expected Uncertainty Large Uncertainty Uncertainty Impact With an ideal surveillance withdrawal and testing schedule and an accurate ETC, the level of uncertainty is known and manageable.
If surveillance testing is delayed and the ETC is inaccurate, the embrittlement uncertainties can be large and unquantified. These uncertainties may impact confidence in the integrity of the reactor pressure vessel for long-term operation.
Ideal capsule withdrawal schedule and accurate ETC Delayed capsule withdrawal schedule and inaccurate ETC
One Source of Uncertainty ETC Underprediction T41J = a measurement of embrittlement representing the shift in transition temperature from brittle to ductile fracture at an impact toughness of 41J or 30 ft-lb.
ETC underprediction starts at a fluence of about 3x1019 n/cm2.
At a fluence of about 6x1019 n/cm2, underprediction becomes relevant.
Deviates from mean Statistically significant
+180oF
-180oF ETC shown in figure found in:
Equation 2 in Regulatory Guide (RG) 1.99, Revision 2, Radiation Embrittlement of Reactor Vessel Materials Equation 3 in 10 CFR 50.61, Fracture toughness requirements for protection against pressurized thermal shock events
Another Source of Uncertainty Lack of Surveillance Data Appendix H to 10 CFR Part 50 requires periodic monitoring of changes in fracture toughness caused by neutron embrittlement. ASTM standard E185-82 allows final capsule fluence to be 2X reactor pressure vessel design fluenceplants change (intended 40-year) design fluence to current license length (e.g., 60 or 80 years).
The ASTM standard permits holding the last capsule without testing.
Capsule withdrawal and testing are repeatedly delayed in some cases to achieve higher fluence.
Impact of Underprediction and Lack of Data PT Limits With a lack of data at high fluence values, the current ETC adjusted with only low fluence data can lead to underprediction of up to 150°F (blue curve).
0 50 100 150 200 250 300 350 400 1.0E+17 2.0E+19 4.0E+19 6.0E+19 8.0E+19 1.0E+20 1.2E+20 1.4E+20 Fluence, n/cm2 Data RG1.99 Fit through Plant data Updated fit with additional data Potential additional data 150°F No correction Fit to original data Embrittlement Fit with potential additional data 75°F Even if the current ETC (orange curve) is adjusted with additional data (yellow curve), underprediction could still be up to 75°F due to the functional form of the ETC.
Improved prediction would require an update to the functional form of the ETC and high fluence surveillance data for validation of the ETC.
Risk-Informed Analysis Risk: Low risk of brittle failure, but uncertainty increases with time.
Safety Margins: Regulations and current licensing basis provide safety margin against brittle failure, but the safety margin decreases as embrittlement uncertainty increases.
Performance Monitoring: Monitoring is needed to ensure analysis results remain valid with time (i.e., no unexpected adverse safety issue occurs).
Safety Margins Performance Monitoring Risk
Assessment and Staff Goals Currently, regulations are sufficient for reasonable assurance of adequate protection against brittle fracture of the reactor vessel.
The staff wants to ensure continued reasonable assurance in long-term operation.
Remedies for the identified issues with reactor pressure vessel surveillance requirements and embrittlement predictions, on a risk-informed, performance-based basis The staff does not want to impact those plants that are not adversely affected by the issues.
Plant-specific surveillance data that cover the end-of-license fluence level Projected fluence at end of license
<~3x1019 n/cm2 Percentage of Fleet Surpassing Fluence Levels Percentage of PWRs Surpassing Fluence Levels Year/Fluence 6x1019 n/cm2 8x1019 n/cm2 6x1019 n/cm2 8x1019 n/cm2 60 years 6%
0%
9%
0%
80 years 22%
10%
34%
15%
With the current state of knowledge, a generalized analysis suggests the overall risk of brittle fracture is low.
The uncertainty in these results is high and increases with time.
Plant-specific details are not considered.
For plants with fluences
>6x1019 n/cm2, safety margins are impacted and are decreasing as uncertainty increases.
High fluence surveillance data are needed to decrease embrittlement uncertainty and validate embrittlement trend predictions.
Plants with fluences
>6x1019 n/cm2 present issues.
Who Is Impacted?
Assessment NRC Staff Goals Embrittlement Underprediction Plant-specific details (e.g., limiting material) may contribute to which plants are impacted.
More work is needed to determine which plants are impacted.
Lack of Surveillance Data Any plant renewing a license that chooses to delay the last capsule may lack proper surveillance data.
Contact Information David RudlandSenior Technical AdvisorDavid.Rudland@nrc.gov David DijamcoMaterials EngineerDavid.Dijamco@nrc.gov